Bad ignition (5)
I believe the answer is:
arson
'bad ignition' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
This is the entire clue.
Can you help me to learn more?
(Other definitions for arson that I've seen before include "serious offence" , "Criminal burning of property" , "matchplay, say?" , "Crime of fire-setting (5)" , "Sonar (anag.)" .)